In some games having an easy mode is a really good feature. In Mass Effect 2 I screwed up the ending a couple times over my various playthroughs and it was really handy to be able to just load an earlier save, set the difficulty to Casual and wade through the enemies like a baws so I could fix my save to import to Mass Effect 3. While I beat ME2 on Insanity and had some really serious head-into-desk moments I appreciate having the option to cruise through the game when I want to. Especially I want to be able to do the game on Veteran difficulty first so I can feel like the story makes sense and save up my dying over and over again until I have seen the whole thing first.
MMOs don't work that way. The problem with MMOs having a variable difficulty is that character progression is the goal everyone is working towards and people are going to set their difficulty level to whatever levels them up faster regardless of what they find fun. Just look at D3 at launch - people did terrible Resplendent Chest runs in Alcarnus for three days straight until the spawn got nerfed even though it was a miserably boring thing to do. The advantage that the Mass Effect titles have in this regard is that progression comes to a stop. You can get to the end slightly faster by going on a easier difficulty but you never feel like you totally wasted your time either way.
What I wonder is if there is any way to build an MMO where variable difficulty levels make sense. Obviously many titles give difficulty choices in endgame dungeons and such but when levelling up there generally isn't any such thing. If the endgame didn't contain progression though I could easily imagine people feeling like they want a challenge and levelling up on hardmode just to have a title or some other such reward to denote their accomplishment. It might seem like it would feel really bizarre to have a world where one person walks up to an orc and cleaves it in two effortlessly and another fights it for a long time, barely to survive, but we already have this with level based systems anyway. Would difficulty setting based challenge really feel any more weird than 'well, that guy has a huge integer above his picture, he can annihilate anything in this part of the world just by looking at it'?
This all relies on an end to the model of progression without limit though. Any time you gate people's content by their progression and make the entire system about getting further along as fast as possible you make difficulty levels make little sense. Nearly everybody will go as fast as possible along the easiest track, complaining all the way that everything is too easy for them. I suspect that endless progression has been a huge part of what has kept WOW so big for so long though so I figure we aren't going to see it end any time soon. I only hope that they get around to making the levelling game a lot more challenging than it was when Cataclysm launched.